In an image-forming process by an electrophotographic system, a visible image is obtained by forming an electrostatic latent image on a photosensitive material comprising an inorganic or organic material, developing the electrostatic latent image with a toner, transferring the developed image onto paper, a plastic film or the like, and fixing the transferred image thereon. The photosensitive material has a positive chargeability or a negative chargeability depending on its constitution, and when leaving an electrostatic image on a part to be printed by light exposure, development is carried out with a reversely charged toner. On the other hand, when carrying out reverse development by destaticizing a part to be printed, development is carried out with the same side charged toner.
A toner comprises a binder resin, a colorant and other additives. A charge control agent is usually added in order to provide satisfactory chargeabilities (including a charging speed, a charging level, a charging stability, etc), a desirable stability as a lapse of time and a satisfactory environmental stability. Properties of the toner are substantially improved by addition of the charge controlling agent.
Today, as a positively triboelectrically chargeable charge control agent known in this technical field, a nigrosine dye, an azine dye, a copper phthalocyanine pigment, a quaternary ammonium salt or a polymer having a quaternary ammonium salt in its side chain is, for example, known. As a negatively triboelectrically chargeable charge control agent, a metal complex salt of a monoazo dye, a metal complex salt of salicylic acid, naphthoic acid or dicarboxylic acid, a copper phthalocyanine dye or a resin containing an acid component is, for example, known.
In the case of a color toner, the market of which is expected to expand in future, a light-colored, preferably colorless, charge control agent which presents no influence over the hue, is indispensable. As a conventional colorless, white or light-colored negatively triboelectrically chargeable charge control agent, a compound utilizing an organic carboxylic acid derivative is known.
However, such a charge control agent is a chromium compound which is problematic against environmental safety which is expected to become more important in future, or a compound having no adequate colorless or light-colored level required for a color toner, or it has had a drawback such that the electrification effect is inadequate, the toner tends to be reversibly electrified, or the dispersibility or the stability of the compound itself is poor.
As a conventional charge control agent, an electrophotographic toner containing a calcium salt of 3,5-di-tert-butylsalicylic acid (e.g. Patent Document 1), a zinc salicylate compound (e.g. Patent Documents 2 to 4) or an aluminum salicylate compound (e.g. Patent Documents 5 to 7) is disclosed.
The charge control agent being a 3,5-di-tert-dibutylsalicylic acid compound disclosed in these Patent Documents, is light-colored or white-colored and contains no heavy metal such as chromium, and thus is applicable to a color toner, and it is a charge control agent, whereby a problem of a heavy metal such as chromium is taken into account. However, it has drawbacks such that the charge-imparting effect is lower than the level required today, the rising speed of electrostatic charge is inadequate, whereby the initial reproduction image tends to be poor in clearness, the quality of the reproduced image is likely to change during continuous copying, or the fluctuation range of the electrostatic charge characteristics of the toner against the environmental conditions such as the temperature, humidity, etc. is large, and the image quality is likely to substantially change due to the seasonal factor or the like. Accordingly, a charge control agent having high charge-imparting effect has been desired.    Patent Document 1: JP-A-62-163061    Patent Document 2: JP-A-63-002074    Patent Document 3: JP-A-63-033755    Patent Document 4: JP-A-4-083262    Patent Document 5: JP-A-63-208865    Patent Document 6: JP-A-63-237065    Patent Document 7: JP-A-64-010261